This invention relates in general to ultrasound diagnostic imaging systems, and in particular, to an adaptive temporal filtering system for enhancing Doppler or time shift signal data.
In conventional color Doppler imaging, an ultrasound system (such as the Acuson XP) derives indices of moving targets inside a body from the received Doppler signals. For example, mean velocity, acceleration, energy and standard deviation (or velocity variance) can be measured from either moving blood cells or moving myocardium or other tissues. These indices are estimated from the received Doppler signal. As with all physical measurements, the estimates include random variations or noise.
In order to reduce the random noise, multiple Doppler samples are acquired and averaged together. By averaging these samples, the random noise tends to cancel out, while the real underlying signals tend to be reinforced, thereby improving the signal-to-noise ratio (SNR). Since the samples are taken sequentially in time, this averaging operation is equivalent to temporal integration of the sampled Doppler signal. Temporal integration, also commonly referred to as persistence, is important for good color Doppler performance since it improves the sensitivity of the system and allows users to detect the very smallest of Doppler signals. While the increase in SNR is important for the conventional color Doppler velocity imaging mode, it is especially important for the more recent color Doppler energy imaging mode where the goal is to detect the very smallest blood flow and perfusion signals.
Typically, temporal integration takes the current Doppler sample containing blood flow information at a location in the body and sums it with a specified number of previous Doppler samples containing blood flow information at the same location in the body at previous times. Sometimes these previous Doppler samples can be "weighted" so that the oldest Doppler samples contribute least to the integrated value. The amount of persistence is a user selectable parameter, and varies either the number of previous Doppler samples included in the integration or the weighting of the previous Doppler samples. In a typical approach, the current persistence function was implemented using a feedback circuit for an input data sequence x(n), the persisted data y(n) may be described by: EQU y(n)=x(n)+a*y(n-1)
where a is a fixed constant selected by the user according to the degree of persistence desired, and n is the sample number.
The old (conventional) temporal integration process is applied uniformly to all received Doppler signals irrespective of other factors such as, for example, their strength. Therefore, as persistence is increased, all aspects of the color Doppler signal are persisted. Here, both weak and strong signals are persisted in the same way. Following are cases illustrating the shortcomings.
First, when a physician moves and re-positions a transducer or when a patient breathes or moves in a clinical environment, very high amplitude Doppler signals are produced from transducer or tissue movements. Additionally, pulsations from the cardiovascular system propagate throughout the body, producing motion artifacts in most tissues. These types of motions create echo amplitudes that are many times greater than those from the weak blood flow signals. The result is color "flash" artifact, where large swaths of color appear on the video display and obscure any low level Doppler signals of interest.
The "flash" artifact is further accentuated by temporal averaging. While the pulse of moving tissue may be of short duration, it will be stretched out in time by the integration process. The old temporal integration process is unable to discriminate against this "flash" artifact and will thus temporally integrate (or persist) the "flash" artifact as long or longer than the blood signal. The long duration of this color "flash" obscures normal scanning and slows down the scanning procedure.
A long temporal integration (or persistence) is often preferred when the clinical user needs to distinguish weak blood perfusion from background noise. However, there are also flow states where the temporal characteristics of the Doppler signal are important, for example, in differentiating steady venous flow from pulsatile arterial flow. There are times when both of these flow conditions occur in the same image. A single persistence cannot be optimized for both of these conditions.
There are also imaging conditions where both good sensitivity and good temporal resolution are required along with reduced "flash" artifact. With the old persistence function, the clinician has to trade off temporal resolution and increased "flash" artifact for better sensitivity. For example, with an indiscriminate persistence process, the high velocity blood flow from a major blood vessel is temporally integrated the same amount as a weak perfusion signal from an adjacent tissue. If the persistence is set correctly for the major vessel flow, the weak perfusion signal may be buried in noise. Alternatively, if the persistence is set correctly for the weak perfusion signals, the high velocity blood flow will appear "sluggish" and its color Doppler representation is not physiological. Furthermore, any "flash" artifact will be persisted too long and could obscure the weak perfusion signals.
Generally, there may be cases where different aspects of the color Doppler signal should be persisted differently.
In view of the above problems of conventional methods, a number of approaches have been proposed. In U.S. Pat. No. 5,357,580, Forestieri et al. propose a temporal filtering scheme which computes a weighted temporal average of the current and previous blood flow velocities, employing weights that are functions of the blood flow velocities, so that imaging information for low velocities are heavily averaged while fast moving blood velocities are maintained with little or no averaging. The output of the system is then displayed in the velocity imaging mode. Velocity may not be the best parameter to use to determine the amount of persistence. Furthermore, velocity imaging is not as sensitive as color Doppler energy imaging, especially for sensing perfusion signals.
Another approach is disclosed in U.S. Pat. No. 5,152,292 to Karp, where the velocity and magnitude components of Doppler signals are compared to "flash" rejection levels, so as to reject and discard velocity components containing "flash" so that such components are not processed by color flow display circuitry. This approach does not adjust the persistence.
None of the above-described approaches is entirely satisfactory. It is therefore desirable to provide an improved temporal filtering system with enhanced characteristics.